The present invention relates to a multicolor image-forming method for forming a full color image of high resolution by irradiation with laser beams. In particular, the present invention relates to a multicolor image-forming method which is useful in the field of printing for forming a color proof (DDCP: direct digital color proof) or a mask image from digital image signals by laser recording.
In the field of graphic arts, printing of a printing plate is performed with a set of color separation films formed from a color original by a lith film. In general, color proofs are formed from color separation films before actual printing work for checking an error in the color separation step and the necessity for color correction. Color proofs are desired to realize high resolution which makes it possible to surely reproduce a half tone image and high performances such as high stability of processing. Further, for obtaining color proofs closely approximating to an actual printed matter, it is preferred to use materials which are used in actual printing as the materials for making color proofs, e.g., the actual printing paper as the base material and pigments as the coloring materials. As the method for forming a color proof, a dry method not using a developing solution is strongly desired.
As the dry method for forming color proofs, a recording system of directly forming color proofs from digital signals has been developed with the spread of electronized system in preprocessing of printing (pre-press field) in recent years. Such electronized system aims at forming in particular high quality color proofs, generally reproducing a dot image of 150 lines/inch or higher. For recording a proof of high image quality from digital signals, laser beams capable of modulation by digital signals and capable of finely diaphragming recording lights are used as recording heads. Therefore, the development of an image-forming material having high recording sensitivity to laser beams and exhibiting high resolution property capable of reproducing highly accurate dots is required.
As the recording material for use in a transfer image-forming method using laser beams, a heat fusion transfer sheet comprising a support having thereon in the order of a light-to-heat converting layer which absorbs laser beams and generates heat, and an image-forming layer which contains a pigment dispersed in components such as a heat fusion type wax and a binder is known (JP-A-5-58045 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)). In an image-forming method using such a recording material, the heat generated at the area of a light-to-heat converting layer irradiated with laser beams fuses an image-forming layer corresponding to the irradiated area, and the fused layer is transferred to an image-receiving sheet arranged on a transfer sheet by lamination, thus a transferred image is formed on the image-receiving sheet.
Further, a heat transfer sheet comprising a support having provided thereon in the order of a light-to-heat converting layer containing a light-to-heat converting material, an extremely thin heat-peeling layer (from 0.03 to 0.3 xcexcm), and an image-forming layer containing a coloring material is disclosed in JP-A-6-219052. In the heat transfer sheet, the bonding strength between the image-forming layer and the light-to-heat converting layer bonded through the intervening heat-peeling layer is reduced by laser beam irradiation, as a result, a highly accurate image is formed on an image-receiving sheet arranged on the heat transfer sheet by lamination. The image-forming method by the heat transfer sheet utilizes so-called ablation, specifically the heat-peeling layer partially decomposes at the area irradiated with laser beams and vaporizes, thereby the bonding strength of the image-forming layer and the light-to-heat converting layer at the irradiated area is reduced and the image-forming layer at that area is transferred to the image-receiving sheet laminated thereon.
These image-forming methods have advantages such that an actual printing paper provided with an image-receiving layer (an adhesion layer) can be used as the material of an image-receiving sheet, a multicolor image can be easily obtained by transferring images different in colors in sequence on the image-receiving sheet, and highly accurate image can be easily obtained. Therefore, these methods are useful for forming a color proof (DDCP: direct digital color proof) or a highly accurate mask image.
For shortening the time required in laser recording when an image is recorded with laser beams, laser beams comprising multi-beams using a plurality of laser beams are used in recent years. When recording is performed using a conventional heat transfer sheet by laser beams of multi-beam, there are cases where the image density of the transferred image formed on an image-receiving sheet is insufficient. The reduction of image density is particularly conspicuous in the case of laser recording with high energy. As a result of the investigation by the present inventors, it was found that the reduction of image density was attributable to transfer unevenness caused by irradiation with high energy laser beams.
Now, there are cases where foreign matters, e.g., dusts, adhere to a recording medium-feeding part, a recording medium-carrying part and printing part in a recording unit and the surface of a recording medium in transportation due to static electricity, and if printing is performed without removing these foreign matters, the foreign matters present between an image-receiving sheet and a heat transfer sheet cause a clear spot and those present between a recording drum and an image-receiving sheet or between a heat transfer sheet and a roller cause image defect such as ring-like unevenness. Accordingly, to remove the foreign matters adhered to the surfaces of the carrying route of a recording medium and a recording part in a recording unit, an adhesive rubber roller (a cleaning roller) is arranged to remove the adhered substance periodically or according to necessity by pressing the adhesive roller against the objective faces of foreign matter removal, to thereby maintain good recording condition.
However, since ordinarily used adhesive rollers are of a type of straight shape of constant roller diameter in the entire width direction, the pressure at the central part of the width direction of the roller weakens due to the deflection of the roller, as a result the degree of adhesion of the roller with recording media lowers and the performance of removing foreign matters is reduced. Further, there is a case where the adhesive force of adhesive rubber materials or adhesive substance used in adhesive rollers lowers by natural aging, therefore, the performance of rollers of removing foreign matters is deteriorated in a couple of months after the production. Due to the reduction of the performance of rubber rollers of removing foreign matters on a printing medium, image defect is sometimes caused such that a desired image is not printed on a recorded image.
On the other hand, there is another problem that by the application of high pressure or by the use of an adhesive roller made of strongly adhesive material, excessive adhesive force is caused and accompanied by peeling of the surface layers such as an image-receiving layer and a cushioning layer of an image-receiving sheet in transportation, and the deviation of the position of an image-receiving sheet fixed on a fixing part of a recording medium, which lead to an image failure of the obtained image.
Accordingly, the present invention aims at solving the problems of the prior art technique and to accomplish the following objects. That is, the objects of the present invention are: 1) a heat transfer sheet can provide excellent sharpness of dots and stability by transfer of a membrane of coloring material, which is not influenced by light sources of illumination as compared with the pigment material and the printed matter, 2) an image-receiving sheet can receive stably and surely the image-forming layer in a heat transfer sheet by laser energy, 3) transfer to actual printing paper can be effected corresponding to the range of from 64 to 157 g/m2 such as art paper (coat paper), mat paper and finely coated paper, delicate texture can be imaged, and a high-key part can be reproduced accurately, and 4) extremely stable transfer releasability can be obtained. A further object of the present invention is to provide a method for forming a multicolor image capable of forming an image having good image quality and stable transfer image density on an image-receiving sheet even when recording is performed by multi-beam laser beams of high energy under different temperature and humidity conditions.
In particular, the object of the present invention is to provide a heat transfer recording method by laser irradiation capable of forming a transfer image having good image quality free of image defects due to foreign matters such as dusts.
A contract proof substituting proofs of an analog style color proof becomes necessary in this film-less CTP (computer-to-plate) age. For obtaining the approval of customers, color reproduction which coincides with the printed matters and analog style color proof is required, and the present inventors have developed DDCP system capable of transfer to actual paper without generating moire by using the same pigment materials as used in the printing inks. That is, this object has been realized by a large sized direct digital color proof (A2/B2) high in approximation to a printed matter and capable of transfer to actual paper and capable of using the same pigment materials as used in the printing inks. The system of the present invention is a system adopting laser membrane transfer, using pigment coloring materials and capable of transferring to actual paper by performing actual dot recording.
The above objects of the pre sent invention have been attained by the following means.
(1) A laser-heat transfer recording method comprising the image-recording steps of feeding an image-receiving sheet having an image-receiving layer and a heat transfer sheet comprising a support having provided thereon at least a light-to-heat converting layer and an image-forming layer to an exposure-recording unit; fixing the image-forming layer in the heat transfer sheet and the image-receiving layer in the image-receiving sheet being superposed vis-a-vis on a recording medium fixing member of the exposure-recording unit; irradiating the heat transfer sheet with laser beams corresponding to image data; and transferring the irradiated area with laser beams of the image-forming layer on the image-receiving layer in the image-receiving sheet; wherein the surface of the image-receiving sheet is cleaned by being brought into contact with an adhesive roll of a crown shape for removing foreign matters, the diameter of the central part of which is larger than the diameters of both ends in the axis direction of the roll body, provided at at least one of the feeding part of the image-receiving sheet and the heat transfer sheet of the exposure-recording unit, a carrying part and a recording part, and the adhesive strength of the image-receiving layer and the underlayer of the image-receiving layer in the image-receiving sheet is from 20 to 100 mN/cm.
(2) The laser-heat transfer recording method as described in the above item (1), wherein the static friction coefficient of the image-receiving layer surface in the image-receiving sheet is 0.7 or less.
(3) The laser-heat transfer recording method as described in the above item (1) or (2), wherein the surface roughness Rz of the image-receiving layer surface in the image-receiving sheet is from 1 to 5 xcexcm.
(4) The laser-heat transfer recording method as described in the above item (1), (2) or (3), wherein pressing controlling members made of a harder material than the material of the adhesive member are provided at both ends of the adhesive roll.
(5) The laser-heat transfer recording method as described in the above item (1), (2), (3) or (4), wherein the cleaning of the surface of the image-receiving sheet by the adhesive roll comprises the steps of fixing the image-receiving sheet on the recording medium-fixing member, cleaning the surface from almost the central part of the relative moving direction of the image-receiving sheet toward one end of the relative moving direction by keeping the adhesive roll in contact with the surface; and thereafter cleaning the surface from almost the central part of the relative moving direction of the image-receiving sheet toward another end of the relative moving direction by keeping the adhesive roll in contact with the surface.
(6) An image-receiving sheet for use in a laser-heat transfer recording unit equipped with an adhesive roll of a crown shape for removing foreign matters, the diameter of the central part of which is larger than the diameters of both ends in the axis direction of the roll body, at at least one of a feeding part of a recording medium, a carrying part and a recording part, wherein the adhesive strength of the image-receiving layer and the underlayer of the image-receiving layer is from 20 to 100 mN/cm.